This invention relates to a surface acoustic wave filter utilizing at least two longitudinal modes which are different from each other and which are excited in the same direction as a propagation direction of a surface wave.
As an example of a surface acoustic wave filter, a surface acoustic wave filter of a longitudinal mode resonator type is disclosed in an article which is contributed by MASAKI TANAKA et al to Fifteenth EM Symposium, Mar. 12, 1986, pages 5-9, and which has a title of "Narrow Bandpass Double Mode SAW Filter". Such a surface acoustic wave filter comprises a piezoelectric substrate, input and output interdigital transducers which are mounted on the piezoelectric substrate such that the output interdigital transducer is located opposite to the input interdigital transducer, and first and second grating reflectors which are mounted on the piezoelectric substrate. The first grating reflector opposes the input interdigital transducer at an opposite side of the output interdigital transducer. The second grating reflector opposes the output interdigital transducer at an opposite side of the input interdigital transducer.
The input interdigital transducer comprises a plurality of input electrode digits which intersect one another. The input interdigital transducer converts an input electric signal into an input surface acoustic wave as a propagated surface acoustic wave. Similarly, the output interdigital transducer comprises a plurality of output electrode digits which intersect one another. The output interdigital transducer receives the propagated surface acoustic wave as a received surface acoustic wave and converts the received surface acoustic wave into an output electric signal.
In such a surface acoustic wave filter, a surface wave excited by the input and the output electrode digits is confined between the first and the second grating reflectors. At this time, a cavity having a predetermined length is formed between the first and the second grating reflectors and the cavity causes a standing wave. Generally, the surface acoustic wave filter utilizes a basic or fundamental mode together with a primary or a secondary mode of the standing wave. Such a surface acoustic wave filter is called a surface acoustic wave filter of a double mode type. The basic, the primary, and the secondary modes have basic, primary, and secondary resonance frequencies, respectively. The primary mode, the secondary mode, and the like are collectively called a higher-order mode. As well known in the art, a resonance frequency of the standing wave is determined by the predetermined length of the cavity. A bandwidth of the surface acoustic wave filter is determined by a frequency interval between the basic resonance frequency and the primary or the secondary resonance frequency.
In order to widen the bandwidth of the surface acoustic wave filter, it is necessary to reduce the number of the electrode digits of the input and the output interdigital transducers. There is, however, a restriction on widening the bandwidth of the surface acoustic wave filter caused by an increase in insertion loss. In other words, the surface acoustic wave filter of a double mode type has problems of increased impedance and degraded out of band attenuation resulting from widening the bandwidth.